1. Field of the Disclosure
The disclosure herein relates to imaging fluids downhole.
2. Description of the Related Art
Oil wells (also referred to as wellbores or boreholes) are drilled into subsurface formations to produce hydrocarbons (oil and gas). A drilling fluid, also referred to as mud, is supplied under pressure to drill the wellbores. A majority of the wellbores are drilled under over-burdened or overpressure conditions, i.e., the pressure gradient in the wellbore due to the weight of the mud column is greater than the natural pressure gradient of the formation in which the wellbore is drilled. Because of the overpressure condition, the mud penetrates into the formation surrounding the wellbore to varying depths, thereby contaminating the natural fluid contained in the formation, which fluid also is referred to herein as the “connate formation fluid” or the “connate fluid.”
To estimate or determine the type or the components of the fluid, including oil, gas and water, in a formation at a particular wellbore depth or to estimate the condition of the reservoir surrounding the wellbore at the particular depth, tools, referred to as the formation evaluation tools, are used during drilling of the wellbore and after the wellbore has been drilled to obtain samples of the connate fluid for analysis. After drilling the wellbore, such tools are conveyed via a wireline or coiled tubing. During drilling of the wellbore, such tools are disposed in a bottomhole assembly above the drill bit, which assembly is conveyed by a drill string that may include a coiled tubing or may be made up of jointed tubulars. To obtain a sample of the connate fluid, a probe is often used to withdraw the fluid from the formation. However, the formation fluid up to a certain distance adjacent the wellbore is contaminated with the mud (i.e., it includes the mud filtrate). Therefore, to obtain a clean sample of the formation fluid, the formation fluid withdrawn from the formation for an initial time period is discarded to ensure that the sample is a clean sample. Various sensors have been used to estimate when the fluid being drawn is clean or of an acceptable quality level, i.e., that the contamination level is acceptable. However, such methods do not provide a visual image of the fluid being withdrawn. Real time visual images can be helpful to an operator for taking samples. Therefore, there is a need for an apparatus and method for obtaining visual images of the fluid downhole.